In optical shape sensing (OSS), the distributed backscattering of light over the entire length of a multi-core optical fiber is measured. The backscattering may come from the naturally occurring Rayleigh scattering or from Bragg gratings written into the fiber (FBGs, Fiber Bragg Gratings). With use of optical interferometric methods, the distributed strain pattern over the entire length and diameter of the optical fiber is interrogated and from that it is possible to reconstruct a 3D shape of the optical fiber, which is useful e.g. when the optical fiber is incorporated in an elongated device such as a medical catheter or guide wire. This requires that the optical fiber has a straight section, called the launch region, with known orientation and position in space as a starting point for a reconstruction of the shape of the optical fiber. This may be achieved by sticky tape or by gluing the starting point on a microscope object side, or by sliding a few centimeters of straight, tightly fitting capillary over the optical fiber. In case the optical fiber is used for tracking position and shape of a medical instrument, such as a guide wire or a catheter, the optical fiber is integrated into the instrument. Integration puts boundary conditions on the manufacturability, placement and stability of the launch region.
Typically, a so-called launch fixture is used to clamp the optical fiber properly as well as to keep track of the position and orientation of the starting point. Small deviations in angle or straightness in the launch region may lead to rather large deviations 1 or 2 meters away from the launch region. E.g. a deviation of 10 micron over 2 cm corresponds to a mismatch of 1 mm at 2 meter, and this example is typical for what is normally considered as just allowable, e.g. for medical application of OSS. It should be noted that clamping of the optical fiber has to be done with care in order to avoid high pressure or uneven distribution of pressure on the launch region because those pressures will change the strain reading of that region compared to the low strain reference values measured originally measured for calibration purposes on the fiber when straightened.
Present launch fixtures are either unpractical, or they simply do not provide the required accuracy, since they are not able to prevent the optical fiber from translating or twisting, while maintaining uniform strain across the launch region. Certain ways of gluing induce creep or are unstable over time, in particular if the instrument, e.g. a catheter or a guide wire, in which the optical fiber is integrated, is operated and stresses are transported to the launch fixture. Sticky tape or Salol are examples of how to make a removable fixation, but both fail to provide the desired stability. Also, tightly fitting of capillary tubing placed around the fiber must be slid over the whole fiber and cannot be applied once the fiber has been terminated, connectorized or integrated in a medical device. Furthermore, it is difficult to assure that the fiber is stress free inside the tube, or that it is fitting correctly. This appears to be true in particular if glue is used to fixate the position and rotation of the fiber.
Thus, none of the existing fixing methods can comply with all of:    1) provide a straight launch region (better than 10 micron over 20 mm for 1 mm tip accuracy),    2) be stable over time and temperature changes,    3) allow easy removal of the optical fiber, and    4) provide no or limited strain exerted on the launch region.